We like this sauce over cooled Caramel Pecan Brownies, but it also makes a rich topping for ice cream.
Stir together the sugar and the water in a 2-quart saucepan over medium heat. Without stirring again, cook until the mixture is dark amber in color, swirling pan carefully and occasionally while cooking, about 20 minutes.
Reduce the heat to low. Slowly and carefully add the cream, stirring with a wooden spoon when the bubbling dies down. Scrape the vanilla seeds into pan; add the pod. Add the lemon juice and butter. Stir to combine. Remove pod.
Cover, and store, refrigerated, up to 1 week. Bring sauce to room temperature, or warm over low heat, before using.